1. Technical Field
The disclosed embodiments generally relate to the field of coatings. In particular, this disclosure relates to coatings as those that may be useful for applying a top layer coating onto a fuser roll used in printing and copying operations. The top layer coating includes a carbon nanotube reinforced fluoropolymer composite of substantially uniform dispersion, where the carbon nanotubes are chemically bonded with the fluoropolymer.
2. Description of the Related Art
In a typical electrostatographic printing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be a photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well known. In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Typically, thermoplastic resin particles are fused to the substrate by heating to a temperature of between about 90° C. to about 160° C. or higher depending upon the softening range of the particular resin used in the toner. It is not desirable, however, to raise the temperature of the substrate substantially higher than about 200° C. because of the tendency of the substrate to discolor at such elevated temperatures, particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time is provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate or belt members. The concurrent transfer of heat and the application of pressure in the nip affect the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member take place during normal operations. Toner particles that offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied there. The referred to “hot offset” occurs when the temperature of the toner is increased to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation to the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface, which has a low surfaced energy to provide the necessary release. To ensure and maintain good release properties of the fuser roll, it has become customary to apply release agents to the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
Fuser and fixing rolls may be prepared by applying one or more layers to a suitable substrate. Cylindrical fuser and fixer rolls, for example, may be prepared by applying an elastomer or fluoroelastomer to an aluminum cylinder. The coated roll is heated to cure the elastomer. Such processing is disclosed, for example, in U.S. Pat. Nos. 5,501,881; 5,512,409; and 5,729,813; the disclosure of each of which is incorporated by reference herein in their entirety.
Fusing systems using fluoroelastomers as surfaces for fuser members are described in U.S. Pat. Nos. 4,264,181; 4,257,699; 4,272,179; and 5,061,965; the disclosure of each of which is incorporated by reference herein in their entirety.
U.S. Pat. No. 5,017,432, which is incorporated by reference herein in its entirety, describes a fusing surface layer obtained from a specific fluoroelastomer, poly(vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene) where the vinylidenefluoride is present in an amount of less than 40 weight percent. This patent further discloses curing the fluoroelastomer with Viton® Curative No. 50 (VC-50) available from E. I. du Pont de Nemours, Inc., which is soluble in a solvent solution of the polymer at low base levels and is readily available at the reactive sites for cross-linking. This patent also discloses use of a metal oxide (such as cupric oxide) in addition to VC-50 for curing.
U.S. Pat. No. 7,127,205, which is incorporated in its entirety herein, provides a process for providing an elastomer surface on a fusing system member. Generally, the process includes forming a solvent solution/dispersion by mixing a fluoroelastomer dissolved in a solvent such as methyl ethyl ketone and methyl isobutyl ketone, a dehydrofluorinating agent such as a base, for example the basic metal oxides, MgO and/or Ca(OH)2, and a nucleophilic curing agent such as VC-50 which incorporates an accelerator and a cross-linking agent, and coating the solvent solution/dispersion onto the substrate. The surface is then stepwise heat cured. Prior to the stepwise heat curing, ball milling is usually performed, for from 2 to 24 hours.
Cross-linked fluoropolymers form elastomers, or fluoroelastomers, are chemically stable and exhibit good release properties. They also relatively soft and display elastic properties. Fillers are often used as in polymer formulations as reinforcing particles to improve the polymer formulation hardness and wear resistance. Thermal conductivity of the fuser system is also important because the fuser or fixer must adequately conduct heat to soften the toner particles for fusing. In order to increase the thermal conductivity of the fuser or fixer member, thermally conductive particles, such as metal oxide particles have been used as fillers. In order to provide high thermal conductivity, the loading of the filler must be high. Loading of a filler that is too high, however, leads to coatings that are too hard, brittle, and more prone to wear. The addition of fillers of conventional metal oxides, such as aluminum, iron, copper, tin and zinc oxides are disclosed in U.S. Pat. Nos. 6,395,444; 6,159,588; 6,114,041; 6,090,491; 6,007,657; 5,998,033; 5,935,712; 5,679,463; and 5,729,813; each of which is incorporated by reference herein in their entirety. Metal oxide fillers, at loadings of up to about 60 wt %, provide thermal conductivities from about 0.2 to about 1.0 Wm−1K1−. However, the increased loading adversely affects the wear and lifetime of the fuser.
A more mechanically robust coating is required for new generation fusing systems in order to improve lifetime and diminish the occurrence of roll failure due to edge wear. Higher thermal conductivity of the top layer would improve heat retention at the surface during fusing, and electrical conductivity would dissipate any static charge buildup.
The disclosure contained herein describes attempts to address one or more of the problems described above.